The horizontal continuous casting process has provided substantial benefits in the metal fabricating arts and is particularly appreciated for its ability to produce extending length constant cross-section castings or billets which would be generally impractical if attempted using other more conventional casting systems. While the structures of horizontal continuous casting systems vary substantially, generally all include a large capacity insulated reservoir for molten metal referred to as a tundish. The most common tundish fabrication includes a container having an interior lining of refractory material chosen for its extreme resistance to the high temperatures associated with metal casting or a modified induction furnace. The tundish is further provided with an outlet orifice near the lower portion of the tundish and provision is made to couple a casting mold in communication with the tundish such that molten metal may be transferred from the tundish interior to the mold.
Mold structures vary somewhat but generally all comprise elongated metal structures defining an input orifice, an outlet orifice and a central passage therethrough. Molds are usually formed of a metal such as copper having a high heat transfer characteristic. The center mold passage is surrounded by a water cooled jacket which in turn is coupled to a supply of cooling water having sufficient flow to carry the heat from the mold passage during the casting process. The molten metal is introduced into the cooled mold via the input orifice and is solidified or frozen within the mold central passage to form the casting. In most instances, a dummy or started bar is inserted into the mold passage and coupled to a casting puller arrangement to initiate the casting process. As the process begins, the started bar is withdrawn from the mold through the mold passage and the metal within the tundish is permitted to flow into the mold passage. Within the mold passage, the casting forms in a welded attachment to the starter bar. The casting pullers thereafter extract the casting formed within the mold in accordance with a predetermined motion profile in which the casting emerges from the mold and is continuously formed or cast as metal within the tundish flows into the casting mold to replace the withdrawn casting.
In most horizontal continuous casting systems, the motion profile used by the puller systems to extract the casting forming with the mold is a series of forward or outward motions interleaved with brief and relatively small reverse motion steps. The latter are generally provided to ensure proper formation of the continuous casting. In many horizontal continuous systems, a slide gate is interposed between the tundish outlet and the casting mold to provide a shut-off valve mechanism. The most common slide gate includes a pair of ceramic plate members which may be interchanged. One ceramic plate defines an aperture therethrough while the other is completely closed defining no aperture. Closure of the slide gate is provided by inserting the plate having no aperture while opening of the slide gate is provided by inserting the plate having an aperture therein.
In endeavoring to improve the horizontal continuous casting process, practitioners have provided a number of variations. For example, U.S. Pat. No. 4,437,509 issued to Ahrens, et al. sets forth a METHOD FOR CONTROL OF BILLET STRIPPING in which the withdrawal of a casting from a cooled horizontal continuous mold involves withdrawing the casting in a series of steps between which the casting is pushed back by a partial step. The backward partial step ensures the welding together of casting sections. A melt breakthrough of the casting skin is prevented by measuring the expansion or contraction of a wall of the mold in contact with the casting and controlling the rate of withdrawal of the casting in response thereto.
U.S. Pat. No. 4,799,533 issued to Ahrens, et al. sets forth an HORIZONTAL CONTINUOUS CASTING MOLD which comprises a mold tube and a pair of flanges. One flange is peripherally disposed of and in abutting, liquid sealing relation with each end of the tube. The sealing relation is preferably provided by a continuous knife-edged protrusion formed in and disposed peripherally of each flange. The knife-edge is adapted to sealingly cooperate with a continuous shallow groove formed in and disposed peripherally of each end of the tube. Cooling means including a liquid jacket and baffle surround the mold tube.
U.S. Pat. No. 4,789,021 issued to Ahrens sets forth a SHORT MOLD FOR CONTINUOUS CASTING having a short taperless casting die coupled to a plurality of cooling plates having means for circulation of coolant therethrough. The cooling plates are arranged in a serially overlapping configuration in which the plates are individually movable to accommodate variations of the casting. Hydraulic means are operative upon the cooling plates to assert a contact force and maintain the cooling plates in contact with the casting surfaces.
U.S. Pat. No. 4,774,996 issued to Ahrens, et al. sets forth a MOVING PLATE CONTINUOUS CASTING AFTERCOOLER having a plurality of cooling plates each of which includes means for circulation of a coolant therethrough. The cooling plates are arranged in a serially overlapping configuration in which the plates are movable to accommodate variations of the casting. Hydraulic means are operative the cooling plates to assert a contact force holding the cooling plates in contact with the surfaces of the casting.
U.S. Pat. No. 5,215,142 issued to Ahrens sets forth a MULTIPLE MOLD WITH CHANGE-OVER FEATURE FOR HORIZONTAL CONTINUOUS CASTING which includes a tundish having a refractory lining and supporting a quantity of molten metal therein. The tundish supports a slide gate shut-off device. A rotating drum supports a plurality of interchangeable casting molds and is operable utilizing a hydraulic motor and linear bearing support. The motor operates to rotate the drum and thereby interchange the casting molds brought into alignment with the slide gate.
Ideally, it would be desirable to operate a horizontal continuous casting system in a virtually endless extended operation in which equilibrium is reached and casting takes place continuously and in which the supply of molten metal with the tundish is periodically replaced. Unfortunately, several factors limit the extent to which a horizontal continuous casting system may be operated in an uninterrupted manner. One of the most significant limitations upon the duration of horizontal continuous casting operation is the substantial wear imposed upon the continuous casting mold. Within the mold, a ceramic element generally known as a break ring is used to interface the slide gate to the mold and is subject to substantial wear during the normal casting process. In addition, the mold itself is usually formed of a copper metal or the like which has a significant tendency to wear as the forming casting is drawn through the mold passage. Other factors such as breakdown or failure of the mold which compromise its integrity or safety also frequently force shutdown of continuous casting operations. In addition to mold problems, however, other factors within the horizontal continuous casting systems of the type to which all mechanical and electromechanical systems are subject, contribute to periodic shutdown of the casting operation.
A substantial number of problems arise when a horizontal continuous casting system is prematurely shutdown which have severe impact upon the economics of casting operation. Shutdown itself is accomplished relatively simply in systems having slide gate apparatus by the insertion of the apertureless ceramic plate which terminates the flow of molten metal into the mold. In other systems not utilizing a slide gate, a starter bar is moved through the mold to the tundish nozzle aperture. The problems associated with such shutdowns, however, are substantial. Almost immediately upon the termination of the casting process, the molten metal within the tundish begins to cool and approach its freezing temperature. This freezing problem is particularly critical in the tundish region near the slide gate and tundish nozzle. The metal freezing within the slide gate and nozzle is virtually impossible to prevent and causes a shutdown. Restarting of the casting operation is impossible and an entirely new setup is necessary.
Thus, while horizontal continuous casting operations may survive brief interruptions of the casting process, any extended duration shutdown forces a complete termination of the casting operation. Thus, once the operator has determined that the interruption of casting operation will force a complete shutdown, the tundish must be completely emptied to prevent the tundish orifice area from freezing off and exacerbating the problems of restarting. In addition, the refractory lining of the tundish must be inspected and repaired in preparation for the next casting operation. The refractory lining within the tundish tends to form small cracks due to its ceramic character during the casting process. These cracks, in turn, become filled with molten metal as the tundish supply of molten metal is maintained. Upon shutdown and cooling, the molten metal within these cracks freezes to form metal “fins” extending in the refractory lining. These metal fins must be extracted from the refractory lining and the lining cracks patched using a repair material before the tundish is again used in the continuous casting operation. In addition, in systems using a slide gate, the ceramic material of the slide gate mechanism is subject to fatigue and cracking and thus must often be replenished and replaced before casting may be resumed.
One of the most critical areas of the horizontal continuous casting apparatus which effects the cycle length between shutdowns is that which includes the tundish nozzle and its interface to the casting mold. The tundish nozzle is an insulative element having a metal flow passage therethrough which is positioned between the discharge orifice of the tundish and the casting mold. The tundish nozzle is typically formed of a material such as zirconium oxide or equivalent material selected for its strength and resistance to heat. One of the critical functions of the tundish nozzle is to provided an insulated interface between the molten metal within the tundish and the cooled surfaces at the entrance of the casting mold or die. The juxtaposition of the high temperature molten metal and the cooled surface of the mold and the mold entrance raises the potential for an undesired freezing of the molten material at the mold input orifice as it passes from the tundish interior through the nozzle passage and into the casting mold. Additionally, the material utilized in constructing tundish nozzles such as zirconium oxide comprises a ceramic material which is pressed, fired, machine ground and further machined to complete its fabrication. The choice of zirconium oxide in the fabrication of tundish nozzles results from a recognition of its relatively low heat transfer characteristic. However, it is also recognized that zirconium oxide is not the optimum material as to wear and mechanical strength. In fact, zirconium oxide often lacks the required strength to provide tundish nozzles which are durable in use and which are not subject to excessive wear. The result is a compromise balancing heat transfer and strength. In many instances, the usable life of the presently manufactured tundish nozzles is as short as three to seven hours with exceptional instances extending for several days of use at the most. Such limitation of the usable life of the presently utilized tundish nozzles imposes a substantial limitation upon the continuous operational capabilities of the present day horizontal continuous casting apparatus.
There remains, therefore, a long felt and unresolved need in the art for evermore improved tundish nozzle structures.